Beam current control circuit for a video camera system

ABSTRACT

A control circuit for a video camera system is used in conjunction with a vidicon tube to maintain constant intensity of the electron beam within such tube. The vidicon utilizes a resistor connected in series with the cathode thereof, and a voltage is developed across this cathode resistor which has a value indicative of beam intensity. This voltage is delivered to the input of a regulator circuit which has an adjustable reference potential selectively adjustable to set the level of electron emission from the cathode of the vidicon. Control of electron beam intensity is accomplished by connecting an output of the regulator circuit to the control grid of the vidicon. The adjustable reference potential is then compared with the voltage across the cathode resistor to regulate the intensity of the electron beam within the vidicon at the preset constant value.

Unite States Patent 91 Caraba et al.

[451 Jan. 2, 1973 [54] BEAM CURRENT CONTROL CIRCUIT FOR A VIDEO CAMERA SYSTEM [75] Inventors: Jerome R. Caraba; Jude P. Schmidt,

' both of Chicago, Ill.

[73] Assignee: Motorola, Inc., Franklin Park, Ill.

[22] Filed: May 24, 1971 [2]] Appl. No.: 146,272

[52] US. Cl ..178/7.2, l78/DlG. 29 [51] Int. Cl. ..H04n 5/34 [58] Field of Search ..l78/7.2, DlG. 29; 315/30 [56] References Cited UNITED STATES PATENTS 3,145,321 8/1964 Sadashige ..]78/7.2 X 2,911,561 11/1959 Fathauer ..178/D1G. 29 3,414,667 12/1968 De Loss Tanner ..178/7.2

Primary Examiner-Robert L. Richardson Attorney-Mueller & Aichele [57] ABSTRACT A control circuit for a video camera system is used in conjunction with a vidicon tube to maintain constant intensity of the electron beam within such tube. The vidicon utilizes a resistor connected in series with the cathode thereof, and a voltage is developed across this cathode resistor which has a value indicative of beam intensity. This voltage is delivered to the input of a regulator circuit which has an adjustable reference potential selectively adjustable to set the level of electron emission from the cathode of the vidicon. Control of electron beam intensity is accomplished by connecting an output of the regulator circuit to the com trol grid of the vidicon. The adjustable reference potential is then compared with the voltage across the cathode resistor to regulate the intensity of the electron beam within the vidicon at the preset constant value.

7 Claims, 3 Drawing Figures PATENTEDJAH 21975 22 UNREGULATED y f NEGATIVE ASSOCIATED PROCESSINGCIR'S.

BEAM AND SUPPLY HIGH VOLTAGE cmcuns FIG 1 \1 VERTICAL 24 g BEAM g8 SWEEP REG VIDICON DETECTION L|7 HORIZONTAL BLANKWG swEEP l5 l4 DETECTIQN 'P SCAN I TERVAL BLANKING INTERVAL FIG. 3

INVENTORS JEROME R. CARABA JUDE P SCHMIDT mzwm ATTYS.

BEAM CURRENT CONTROL CIRCUIT FOR A VIDEO CAMERA SYSTEM BACKGROUND OF THE INVENTION transconductance device. Generally, such transconm ductance devices have a target area on which is displayed a visual scene to be electronically reproduced for transmission. The target area may include either a photosensitive coating or a mosaic coating depending on the type of device involved, and this coating on the target area produces a variable signal output in response to variations in light intensity caused by the scene displayed thereon. When the target area, with the scene thereon, is scanned by a constant intensity electron beam, a modulated output signal will be developed at the output of the vidicon. For proper operation the electron beam intensity is required to be maintained substantially constant. Should variations in electron beam intensity occur, a false variation in the output signal of the tube will also occur and this false signal will produce an error which is not part of the scene displayed on the target area.

Heretofore, control of electron beam intensity of camera tubes has been accomplished by voltage regulation of a bias potential or of a large negative power supply, and was independent of feedback signals from the vidicon tube. This type of voltage regulation is independent of current emission from the cathode of the vidicon and therefore can not sense inadvertent current changes of the electron beam when sweeping or scanning the scene on the target area. Also, it is relatively expensive to regulate larger power supplies.

SUMMARY OF THE INVENTION device used in a video camera system, wherein feedback is provided to the voltage regulator in a closed loop configuration from the transconductance device.

Another object of this invention is to provide an improved control circuit for a video camera system which uses cathode current of the video tube to control the grid voltage applied to the video tube.

A further object of this invention is to provide a control circuit for a video camera system which is readily adjustable for use with a sweep blanking circuit.

Yet another object of this invention is to provide an improved control circuit for a video camera system which will eliminate the need of large and expensive power supply regulating systems.

Briefly, the control circuit of this invention contemplates the use of a current responsive means for sensing the amount of electron beam current passing through a vidicon, this being primarily the cathode current of the vidicon. The current responsive means is directly coupled to a beam intensity voltage regulator circuit which has incorporated therein a reference potential setting device to produce an adjustable signal which is compared with a signal produced by the current responsive means. The regulator circuit operates to produce an output signal which will control the bias voltage at the grid electrode of the vidicon. This voltage control of the grid electrode will in turn control the electron beam intensity from the cathode. Increase in cathode current will cause the regulator circuit to produce a decreasing effect at the control grid, and conversely, decrease in the cathode current will cause the regulator circuit to produce an increasing effect at the control grid. The close loop circuit arrangement of this invention is of low cost design, and as such, has a particular utility for use in low cost television cameras of the surveillance and/or portable types. A simple blanking circuit is connected at the input of the voltage regulator circuit and will prevent blanking pulses from reaching the regulator so that the regulator will not respond to the normal off or blanking condition of the cathode current. In addition to this, the positiveinstantaneous voltage that appears at the vidicon cathode during the blanking intervahdrives the vidicon into its cut-off region thus preventing retrace lines from appearing in the video presentation.

BRIEF DESCRIPTION OF THE DRAWINGS age regulator in response to scanning signals and blanking pulses produced for each sweep cycle of the vidicon tube.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENT Referring now to FIG. 1, there is seen a video camera system which is designated generally by reference numeral 10. The video camera system 10 includes a transconductance device 12, such as a vidicon, for electronically reproducing a visual scene in a manner well known in the art. Although the invention herein is illustrated as having particular utility when used in conjunction with vidicon tubes, it will be understood that any suitable transconductance device, such as a plumbicon, iconoscope, or other video camera device, can be used.

A current responsive means, here illustrated as a resistor 14, is connected to the cathode electrode of the vidicon 12 to produce a voltage indicative of the amount of electron current passing from the cathode to the other elements in the form of an electron beam within-the vidicon. The voltage developed across the resistor M is compared with an adjustable reference potential which is developed across a potentiometer 15, or other suitable adjustable device. The adjustable reference potential across the potentiometer 14 is part of a voltage regulator circuit M which has an input terminal l'i thereofdirectly coupled to the resistor 14. Thus, the voltage regulator 16 produces an output signal at an output terminal 18 in response to the difference in the potentials across the resistor 14 and at the potentiometer 15. The voltage at output 18 is then delivered to a control grid of the vidicon, to bias the same and control the amount of electron emission from the cathode. This action will control the electron beam intensity.

The output signal from the vidicon 12 is delivered to a video amplifier, and its associated processing circuitry 20 which may also include the high voltage circuits necessary for normal operation. These particular circuit functions do not form part of this invention, and are here illustrated only for completeness and clarity.

The circuit 20 receives video signal information which is developed in response to the modulated output current of the vidicon and processes the video signal information by conventional techniques, well known in the art.

A power supply 22 is connected to the voltage regulator 16 to deliver operating potential thereto which, in turn, may also provide a bias voltage supplied to the vidicon 12. To insure that the vidicon is protected in the event of either horizontal or'vertical sweep failure, disabling inputs from the vertical sweep detection circuitry 24 and horizontal sweep detection circuitry 26 are applied to the beam regulator at a common input line 29 in addition to the composite blanking. These inputs are combined at a common input line 29 where the voltage regulator circuit 16 is temporarily disabled during the retrace portion of each sweep cycle so that the blanking signals prevent any significant change in the control grid voltage during the cut-off period of the vidicon.

Referring now to FIGS. 2 and 3, there is seen a detailed schematic diagram of the voltage regulator circuit of this invention and a waveform illustrating the blanking operation of the circuit. Similar components in FIG. 2 are suffixed by letter (a) and are intended to correspond to the same numbered component in FIG. 1.

The voltage regulator circuit 16a includes a transistor 46 which regulates the intensity of beam current passing through the cathode 120. This is accomplished by a direct connection of the collector electrode 460 of the transistor 46 to the control grid 12g of the vidicon 12a. The voltage applied to the emitter of transistor 46 is the variable reference potential which is developed by the potentiometer a, and thus sets the operating bias conditions of transistor 46. By setting the potentiometer 15a, transistor 46 will conduct to a given level dependent upon the voltage across resistor 14a and set the bias level at grid 12g. During operation of the vidicon 12a the cathode current will be maintained in accordance with the setting of potentiometer 15a of the beam regulator circuit 16a. That is, if cathode current increases, the voltage drop across resistor 14a also increases and the conduction to transistor 46 decreases as a result of the decreased bias on the base-emitter junction thereof. This decrease in conduction of transistor 46 causes a decrease, in the voltage'drop developed across a resistor 48. The resistor 48 is connected to a negative voltage supply and therefore applies a more negative voltage to the grid electrode 123 to decrease the-current emission from the cathode 120. This will decrease the intensity of the electron beam within the vidicon and thus regulate the beam current.

Potentiometer 150 from which the adjustable reference potential is derived forms part of a voltage divider network which includes a resistor 50 connected to a 8+ potential at terminal 51. To provide temperature compensation of the voltage regulator circuit 16a a pair of diodes 52 may be connected in series with the potentiometer 15a and are of opposite conductivity with respect to the base emitter junction of transistor 46. The pair of diodes 52 may be replaced by a single diode if desired or eliminated if performance over a wide temperature range is not required. This provides ambient temperature tracking of the transistor 46 in that an increase in temperature will cause a negative voltage increase at the base to emitter junction of the transistor and a corresponding positive voltage increase at the anode to cathode junctions of the diodes 52 and. these two voltages will counteract and cancel each other. Resistor 44 (optional) also serves to apply a bias voltage at the base of transistor 46 and provides a leakage current path for the collector base leakage current of the transistor. A diode (optional, which is only required if resistor 44 is used) 54 is connected across the base emitter junction of transistor 46 to provide for protection of the transistor in case of high voltage transients to prevent reverse voltage breakdown of the transistor and to protect the transistor during the blanking operation. A filter capacitor 56 may be coupled between the collector electrode of transistor 46 and ground potential to eliminate extraneous signal information from being applied to the grid electrode 12g. Under normal operating conditions, a slight reduction in beam current occurs during the top few lines of each field after blanking. However, in very critical applications, where extremely uniform beam current over the entire field is required, a capacitor 45 may be connected between the base of transistor 46 and ground. The circuit arrangement illustrated in FIG. 2 is capable of uniform operation over a wide temperature range in the order of 30C to +C.

The waveform of FIG. 3 illustrates the blanking pulses 32 spaced by intervals 30 which represent the normal scan signals which are developed during sweep' operation of the electron beam within the vidicon 12a. A transistor input circuit 35 is connected to the input line 17a and includes a transistor 36 and a diode 38 having its cathode electrode connected to a junction 40. A current limiting resistor 37 is connected between the 13+ terminal 39 and the collector of transistor 36. The junction 40 is a common junction for the current sensing resistor 14a and an input diode 42 of the beam regulator 16a. During the unblanked portion of the scanning cycle, a positive-voltage is applied to the base electrode 36b of transistor 36 to maintain the transistor in a conductivestate. The conduction of transistor 36 causes a reverse bias condition to be applied to the anode of diode 38 and, as a result, the cathode 12c of the vidicon 12a is maintained at a voltage level determined by the resistance value of the resistor 14a, and the vidicon cathode current. One feature of the regulator circuit of this invention is that if a sweep signal fails to occur at the base electrode 461; of the transistor 36, transistor 36 will be rendered nonconductive and the diode 38 will be forward biased to apply a full B+ potential from terminal 39 to the junction 40 and therefrom to the cathode of the vidicon 12a. This high positive potential on the cathode 12c will render the vidicon inoperative, i.e., no electron emission from the cathode. Any blanking pulses applied to the base electrode 36b of transistor 36 will render the transistor nonconductive to cause the full forward bias from terminal 39 to be applied through diode 38, and disable the cathode 12c as indicated by the pulse 32, of FIG. 3. This operation is accomplished by the combination of vertical sweep detection circuit 24, horizontal sweep detection circuit 26 and blanking adder circuit 28 of FIG. 1. The beam remains cut-off for the duration of the blanking pulse so that no video signal is developed during the retrace operation and the blanking pulse is prevented from interfering with the regulator circuit 16a by means of the blocking diode 42.

Such closed loop voltage regulation is advantageous in that any malfunction of a component part which would tend to cause an increase in current flow of the vidicon will be sensed by the resistor 1 or 14a and delivered to the beam regulator which, in turn, will develop an output signal to counteract such increased current flow. Similarly, decreases in electron beam intensity will be compensated for by the voltage regulator circuit 16 to maintain the electron beam substantially constant and thus accurately electrically reproduce the visual scene displayed on the target area.

What has been described is a simple and inexpensive control circuit for a video camera system for controlling the electron beam intensity of a vidicon tube, or other transconductance device as the case may be.

We claim:

1. A control circuit for a video camera system having an electron beam transconductance device with a cathode and a control. grid for controlling an electron beam to scan a target screen, comprising in combination, current responsive means connected in circuit with the cathode of the transconductance device for sensing the electron beam current flowing through the cathode and producing a signal in response thereto, a voltage regulator circuit having an input terminal and an output terminal and providing a voltage at said output terminal which varies in response to the signal applied to said input terminal, means connecting said input terminal to said current responsive means for applying said signal thereto, circuit means connecting said output terminal of said voltage regulator circuit to the control grid of the transconductance device to control the bias applied thereto, and variable reference potential means connected in circuit with said voltage regulator circuit for producing a reference signal for comparison with said signal produced by said current responsive means, said voltage regulator circuit controlling the bias applied to said control grid to maintain the electron beam current produced by the cathode at a predetermined constant value'determined by the setting of said reference potential means.

2. The control circuit for a video camera system according to claim 1 further including a diode connected in series with said variable reference potential 'means and wherein said voltage regulator circuit includes a transistor having a diode junction connected to said variable reference potential means, with said diode compensating for the variation of the voltage across said diode junction with temperature, whereby temperaturecompensation is achieved by the opposite conductivity junctions of said diode and said transistor.

3. The control circuit for a video camera system according to claim I wherein said voltage regulator circuit includes a transistor having the emitter electrode thereof con ected to said variable reference otential means and t e base electrode thereof connect d to said current responsive means, and the level of conduction of said transistor is controlled by the difference in potential between said adjustable signal produced by said variable reference potential means and said signal produced by said current responsive means.

4. The control circuit for a video camera system according to claim 1 wherein said voltage regulator circuit includes a transistor having the emitter electrode thereof coupled to said variable reference potential means and the collector electrode thereof coupled to the control grid of the transconductance device, and further including blanking circuit means, a diode connected between said blanking circuit means and the base electrode of said transistor, with said current responsive means being connected to the junction of said diode and said blanking circuit means, said diode being forward biased during a normal scan sweep of the electron beam within the transconductance device and reverse biased during a blanking condition of the electron beam.

5. A control circuit for a video camera system, including in combination, a vidicon having a target area for receiving a visual scene projected thereon to be electronically reproduced by successive scanning operation of an electron beam, said vidicon further having a cathode for producing an electron beam and a control grid for controlling the intensity of said electron beam, amplifier means connected to said vidicon for receiving video signal information produced during each sweep of the electron beam over said target area, regulator circuit means including a semiconductor device having an input electrode coupled to the cathode of said vidicon and an output electrode coupled to the control grid of said vidicon, and a current responsive element connected in circuit with the cathode of said vidicon to produce a control signal indicative of the intensity of said electron beam, said control signal being sensed at said input electrode of said semiconductor device of said regulator circuit means to then control the bias voltage applied to the control grid of said vidicon and maintain said electron beam at a desired predetermined constant level.

6. The control circuit for a video camera system according to claim 5 further including blanking circuit means connected to said regulator circuit means, said blanking circuit means including means responsive to a vertical sweep duration of said electron beam and means responsive to a horizontal sweep duration of said electron beam to blank the electron beam during periods between each sweep cycle over said target area.

7. The control circuit for a video camera system according to claim 5 further including an adjustable reference means connected in circuit with said regulator circuit means to set the operating level of said regulator circuit means so that the intensity of the electron beam within said vidicon is maintained at the predetermined constant level. 

1. A control circuit for a video camera system having an electron beam transconductance device with a cathode and a control grid for controlling an electron beam to scan a target screen, comprising in combination, current responsive means connected in circuit with the cathode of the transconductance device for sensing the electron beam current flowing through the cathode and producing a signal in response thereto, a voltage regulator circuit having an input terminal and an output terminal and providing a voltage at said output terminal which varies in response to the signal applied to said input terminal, means connecting said input terminal to said current responsive means for applying said signal thereto, circuit means connecting said output terminal of said voltage regulator circuit to the control grid of the transconductance device to control the bias applied thereto, and variable reference potential means connected in circuit with said voltage regulator circuit for producing a reference signal for comparison with said signal produced by said current responsive means, said voltage regulator circuit controlling the bias applied to said control grid to maintain the electron beam current produced by the cathode at a predetermined constant value determined by the setting of said reference potential means.
 2. The control circuit for a video camera system according to claim 1 further including a diode connected in series with said variable reference potential means and wherein said voltage regulator circuit includes a transistor having a diode junction connected to said variable reference potential means, with said diode compensating for the variation of the voltage across said diode junction with temperature, whereby temperature compensation is achieved by the opposite conductivity junctions of said diode and said transistor.
 3. The control circuit for a video camera system according to claim 1 wherein said voltage regulator circuit includes a transistor having the emitter electrode thereof connected to said variable reference potential means and the base electrode thereof connected to said current responsive means, and the level of conduction of said transistor is controlled by the difference in potential between said adjustable signal produced by said variable reference potential means and said signal produced by said current responsive means.
 4. The control circuit for a video camera system according to claim 1 wherein said voltage regulator circuit includes a transistor having the emitter electrode thereof coupled to said variable reference potential means and the collector electrode thereof coupled to the control grid of the transconductance device, and further including blanking circuit means, a diode connected between said blanking circuit means and the base electrode of said transistor, with said current responsive means being connected to the junction of said diode and said blanking circuit means, said diode being forward biased during a normal scan sweep of the electron beam within the transconductance device and reverse biased during a blanking condition of the electron beam.
 5. A control circuit for a video camera system, including in combination, a vidicon having a target area for receiving a visual scene projected thereon to be electronically reproduced by successive scanning operation of an electron beam, said vidicon further having a cathode for producing an electron beam and a control grid for controlling the intensity of said electron beam, amplifier means connected to said vidicon for receiving video signal information produced during each sweep of the electron beam over said target area, regulator circuit means including a semiconductor device having an input electrode coupled to the cathode of said vidicon and an output electrode coupled to the control griD of said vidicon, and a current responsive element connected in circuit with the cathode of said vidicon to produce a control signal indicative of the intensity of said electron beam, said control signal being sensed at said input electrode of said semiconductor device of said regulator circuit means to then control the bias voltage applied to the control grid of said vidicon and maintain said electron beam at a desired predetermined constant level.
 6. The control circuit for a video camera system according to claim 5 further including blanking circuit means connected to said regulator circuit means, said blanking circuit means including means responsive to a vertical sweep duration of said electron beam and means responsive to a horizontal sweep duration of said electron beam to blank the electron beam during periods between each sweep cycle over said target area.
 7. The control circuit for a video camera system according to claim 5 further including an adjustable reference means connected in circuit with said regulator circuit means to set the operating level of said regulator circuit means so that the intensity of the electron beam within said vidicon is maintained at the predetermined constant level. 